Radiocommunication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radiocommunication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radiocommunication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radiocommunication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radiocommunication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LIE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
The LTE Rel-8 standard has recently been standardized, supporting bandwidths up to 20 MHz. However, in order to meet the upcoming IMT-Advanced requirements, 3GPP has initiated work on LTE-Advanced. One aspect of LTE-Advanced is to support bandwidths larger than 20 MHz in a manner which assures backward compatibility with LTE Rel-8, including spectrum compatibility. This implies that an LTE-Advanced carrier, which is wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a “component carrier”.
For early LTE-Advanced deployments, it is expected that there will be a smaller number of LTE-Advanced-capable terminals in operation as compared to many LTE legacy terminals in operation. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE-Advanced carrier. One way to achieve this objective is by means of carrier aggregation. Carrier aggregation implies that, for example, an LTE-Advanced terminal can receive multiple component carriers, where the component carriers have, or at least have the possibility to have, the same structure as a Rel-8 carrier. An example of carrier aggregation is illustrated in FIG. 1, wherein five 20 MHz component carriers 10 are aggregated to form a single wideband carrier.
LTE systems use hybrid-ARQ where, after receiving downlink data in a subframe, the terminal attempts to decode that data and reports to the base station whether the decoding was successful (ACK) or not (NAK). In the case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data. Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information on which terminals are supposed to receive data and upon which resources in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2 or 3 OFDM symbols in each subframe. A terminal will thus listen to the control channel and, if it detects a downlink assignment addressed to it, the terminal will decode the data and generate feedback in response to the transmission in the form of an ACK or a NAK depending on whether the data was decoded correctly or not.
The HARQ protocol employed in LTE systems uses a number of HARQ processes each having their own identification (ID), where a HARQ process is essentially a pointer to a logical buffer in the receiver. When retransmissions are performed for a higher layer PDU, they are transmitted in the same HARQ process and the receiver knows (from the HARQ process ID) that the retransmissions should be combined with each other. When the transmitter has received an ACK for the transmitted data it can start sending a new transmission in the HARQ process and indicates that to the receiver with a new data indicator on the L1/2 control channel. A stop and wait protocol is used for each HARQ process but since transmissions can be ongoing in multiple, staggered HARQ processes simultaneously a continuous transmission is possible. The number of HARQ processes needed to achieve a continuous transmission depends on, among other things, the processing requirements in the eNodeB and the user equipment (UE). For LTE systems, about 8 HARQ processes are needed to provide frequency division duplex (FDD) operation.
One possibility for implementing carrier aggregation is to perform coding and hybrid-ARQ retransmissions on a per component carrier basis. An example of this type of carrier aggregation is illustrated in FIG. 1, where data to be transmitted to a given terminal is transmitted on three component carriers 20, 22 and 24. In the existing LTE structure, this technique would correspond to having a transport block (or two transport blocks in case of spatial multiplexing) per component carrier. The structure in FIG. 2 uses multiple, independent hybrid-ARQ entities 26, 28 and 30 to implement the HARQ processes in the medium access control (MAC) layer. For hybrid-ARQ operation, acknowledgements informing the transmitter regarding whether the reception of a transport block was successful or not are needed. One way to implement such acknowledgements would be to transmit multiple acknowledgement messages, e.g., one per component carrier (in the case where spatial multiplexing is employed, an acknowledgement message would correspond to two bits as there are two transport blocks on a component carrier in the first release of LTE, however in the absence of spatial multiplexing, an acknowledgement message is a single bit as there is only a single transport block per component carrier), but also other implementations are possible.
If carrier aggregation is performed as shown, for example, in FIG. 2, this means that several transport blocks (TBs) may be transmitted per transmission time interval (TTI). If, for example, 20 MHz carriers are aggregated to a total bandwidth of 100 MHz to achieve a 1 Gbps peak rate, 5 TBs per TTI need to be transmitted. If spatial multiplexing is used this increases to 10 TBs per TTI with the current LTE solution, which uses two TBs per TTI when performing spatial multiplexing. If the solution for spatial multiplexing is modified in future releases, the number of TBs per TTI may increase even further. With the current protocol structure in LTE, e.g., as described in the standards specification 3GPP 36.322 entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Link Control (RLC) protocol specification”, where one RLC PDU is transmitted per TB this implies that many RLC PDUs may need to be transmitted per TTI, which quickly consumes RLC sequence numbers (e.g., 10 sequence numbers per TTI). RLC segmentation for LTE is discussed in more detail below. Thus carrier aggregation may, in some situations, result in the undesirable result of stalling of the RLC protocol since only half of the sequence number (SN) space may be outstanding (transmitted but not yet acknowledged) at any given time to ensure that there is no SN ambiguity between a new transmission and a retransmission for a particular SN.